The present invention relates to methods and apparatus for detecting failures made by a wire bonding system which forms a wire bond between two contact surfaces. For example, one contact surface may be a pad of a die and the other contact surface may be a point on a substrate (the term xe2x80x9csubstratexe2x80x9d will be used in this document broadly to include a substrate on which a die is mounted or a lead frame), a different pad of the same die, or a pad of a second die.
It is known to equip an automatic wire bonding machine with a device to detect defects which arise during the wire bonding process. This is particularly necessary for the first bond on the die and for the second bond on the substrate. There are several known ways to detect defects.
One possible way, in the case that the bonding machine employs ultrasonic bonding, is to monitor during the bonding process one or more signals from the ultrasonic system, and to deduce from a comparison between waveforms of the ongoing bond with waveforms of known good bonds whether a failure has occurred or not (U.S. Pat. No. 5,889,210).
Another method is to inject a DC voltage signal into the bond wire and to monitor changes of this signal during and/or after the bonding process (U.S. Pat. Nos. 4,555,052; 5,058,797). This method essentially evaluates changes in the impedance between the bond wire and a reference point, which is usually the chassis ground of the machine.
By contrast, U.S. Pat. No. 4,586,642 electrically couples an oscillator to the bond wire. During the bonding process the capacitance of the bonded system changes, which changes the frequency of the signal. Bonding failures are observed from measurements of the frequency at various stages of the operation.
There are a great variety of die and substrate designs on the market and the requirements on the properties of the injected signal, and the changes which this signal undergoes when it is connected to the die or the substrate, vary over a wide range. Semiconductor manufacturers are generally concerned that the signal applied might cause degradation or even damage to the chip if the signal amplitude is too high. On the other hand, some components, such as optical devices, require a higher signal amplitude because of the relatively high forward voltage drop, which is inherent to this technology. The impedance between the connection point of the bond wire and the reference point also varies over a wide range depending upon the circuitry on the microelectronic chip and the technology employed to implement this circuitry. It may change between different bond pads on the same die and also depend on the bonding sequence.
Traditionally, lead frames used to be made of conductive material and were connected to chassis ground of the machine over the mechanical parts that hold the lead frames in place. In recent years, however, a substantial percentage of designs have been introduced where the traditional conductive lead frame has been replaced by a non-conductive substrate. With this type of substrate it is more difficult to perform a bond failure detection.
The present invention aims to provide a new and useful method and apparatus for detecting bonding faults, and suitable for use with a large variety of circuits to be processed.
In general terms, the present invention proposes that a monitoring device forms an electric circuit including the bond wire, measures an electric signal at a point in the circuit during at least one stage of a bonding process, and generates from the measurement an output which discriminates between successful and unsuccessful bonding. The monitoring device employs at least one variable parameter during the measuring and/or discriminating process. The value of the variable parameter(s) is determined beforehand by monitoring one or more examples of actual wire bonding operations (including successful examples) to find parameter value(s) associated with successful bonding,
Thus, the detection method may adapt to changing requirements automatically and operate under optimal conditions for a large variety of circuits to be processed, e.g. for any given die or substrate.
The present invention is applicable for measuring bonding faults of bond wire which connects a first contact surface (a particular example is a die pad) to a second contact surface. Typically, the second contact surface is a surface of a xe2x80x9csubstratexe2x80x9d as defined above, e.g. a lead frame. Alternatively, the second contact surface may be another die pad (a possibility mentioned in U.S. Pat. No. 5,328,079). The words xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d as used here do not necessarily imply an order for the bonding processes.
Preferably the monitoring process includes applying an oscillating voltage to a combination of a load and the bond wire in series, and analysing a voltage derived from the bond wire during stages of the bonding process. Using an AC signal, rather than a DC signal, allows the detection method to be applied to a wider range of devices than is possible using the prior art devices described above which employ a DC signal. This is because the chip (die) impedance (ZD) is not purely resistive, but also has a capacitive component, and because there is a certain capacitance between the silicon die and the reference point. This is advantageous if the resistive component of ZD is very high, which is in particular the case when the substrate is made of nonconductive material.
Preferably, the measurement includes a measurement of an amplitude of the voltage signal (e.g. peak amplitude or RMS amplitude). Preferably the frequency of the voltage signal is predetermined, or is controlled by the monitoring device, rather than being a measured parameter.
In some embodiments, the monitoring device measures the phase of the voltage signals, instead of, or in addition to, their amplitude.
The invention may be used to monitor a wiring process including in which each of a plurality of first contact surfaces are connected to respective second contact surfaces. In this case, optimal parameters may be found for each pair of contact surfaces (e.g. for each bond pad on the die and each bonding point on a substrate). Alternatively, the process of forming any combination of these bonds may be performed using the same parameters, if it is determined that the accuracy of the monitoring is not significantly reduced.
The values of the parameters may be stored in memory and recalled when the particular wire connection is to be monitored.